CBSE Class 9 · The Price Puzzle: What Drives the Market

Demand and Supply Class 9: Laws, Schedules and Determinants

Updated 17 October 2026 · Fact-checked

Demand is the quantity buyers are willing and able to buy at a price; the law of demand says that as price rises, quantity demanded falls (inverse relationship). Supply is the quantity sellers offer at a price; the law of supply says that as price rises, quantity supplied rises (direct relationship). Market demand and supply are the sums of individual demands and supplies, and factors other than own price can shift them.

Understand Demand and Supply: Laws, Schedules and Determinants

Demand. The quantity of a product people are willing and able to buy at a particular price. A wish is not demand; it must be backed by purchasing power.

Law of demand. When the price rises, quantity demanded falls, and when the price falls, quantity demanded rises, other things constant. Plot price on the y-axis and quantity on the x-axis to get a downward-sloping demand curve. *Diminishing marginal utility* helps explain it: each extra unit gives less additional satisfaction, so buyers will pay less for more units.

NCERT's mango schedule for Srivalli: at Rs 150 per kg she buys 1 kg, at Rs 100 she buys 2 kg, at Rs 50 she buys 3 kg.

Market demand is the sum of all individual demands at each price. With Srivalli, Alex and Israt:

Price	Srivalli	Alex	Israt	Market demand
Rs 150	1	2	3	6 kg
Rs 100	2	4	6	12 kg
Rs 50	3	6	9	18 kg

The market curve is flatter than an individual's: a price fall from Rs 150 to Rs 50 raises Srivalli's purchase by 2 kg but the market's by 12 kg.

Other determinants of demand (these shift the whole curve):

Prices of related goods: *substitutes* replace each other (tea and coffee), so if coffee gets dearer, demand for tea rises; *complements* are used together (cinema tickets and popcorn), so if tickets get dearer, demand for popcorn falls.
Income: higher income usually raises demand or shifts buyers to better quality.
Tastes and preferences.
Population size and composition: more children raise demand for sports shoes; more elderly people, for orthopaedic shoes.
Seasonality: sweaters in winter, sweets at festivals.
Future price expectations: if a fall is expected, buyers postpone; if a rise is expected, they buy now.

Supply. The quantity sellers are willing and able to offer at a price.

Law of supply. As price rises, quantity supplied rises: a direct relationship and an upward-sloping supply curve. Higher prices raise profit, encourage producers to produce more and attract new firms. Seller A offers 1 kg at Rs 50, 2 kg at Rs 100 and 3 kg at Rs 150.

Market supply is the sum of individual supplies:

Price	A	B	C	Market supply
Rs 50	1	3	2	6 kg
Rs 100	2	4	6	12 kg
Rs 150	3	7	8	18 kg

Other determinants of supply: prices of related goods (a farmer grows more chickpeas if wheat prices fall and chickpea prices rise), number of sellers, technology (drip irrigation, cold storage), future expectations (potato wholesalers expecting higher prices may hold back supply now), and also input costs and weather.

Change in quantity vs change in demand. A change in the good's own price moves you *along* the curve; a change in any other determinant *shifts* the whole curve.

Demand and Supply: Laws, Schedules and Determinants: key facts

Law of demand: Price up, quantity demanded down
Inverse relationship
Demand curve slopes downward; Other factors constant.
Law of supply: Price up, quantity supplied up
Direct relationship
Supply curve slopes upward; Higher profit attracts producers.
Market totals: Market demand = sum of individual demands at each price | Market supply = sum of individual supplies at each price; Add across buyers or sellers.
Related goods: Substitute price up: demand for the other up | Complement price up: demand for the other down; Tea-coffee; ticket-popcorn.
Demand determinants: Related goods
Income
Tastes
Population
Seasonality
Future price expectations; Shift the curve.
Supply determinants: Related goods
Number of sellers
Technology
Future expectations
Input costs, weather; Shift the curve.

How to answer Demand and Supply: Laws, Schedules and Determinants questions

Questions ask you to define demand and supply, state the laws, add individual schedules to get market totals, classify goods as substitutes or complements, and say whether an event shifts demand or supply.

1For a schedule, add the individual quantities row by row, price by price, and check the totals.
2For an event, identify whether it affects buyers (demand) or sellers (supply).
3Decide the direction: a rise in a substitute's price raises demand; a rise in a complement's price lowers demand.
4Distinguish a move along the curve (own price changed) from a shift of the curve (another determinant changed).
5State the law in words and end with the curve's slope.

Demand and Supply: Laws, Schedules and Determinants in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Umbrellas in the monsoon

Suppose umbrella sellers see sales jump when monsoon rains begin, although the prices have not changed.

Is this a change in quantity demanded or in demand? Which determinant is it?

Try it first, then show the solutionSolution

The own price is unchanged, so it is not a move along the curve.
The curve shifts rightwards.
The determinant is seasonality.

Answer: It is a change in demand (a rightward shift), caused by seasonality: more people want umbrellas at every price.

Memory hook: Season moves the curve; price moves along it.

A two-buyer market

Suppose at Rs 20 per dozen eggs, buyer M wants 2 dozen and buyer N wants 3 dozen; at Rs 10, M wants 4 dozen and N wants 6 dozen.

Find market demand at each price.

Try it first, then show the solutionSolution

At Rs 20: 2 + 3 = 5 dozen.
At Rs 10: 4 + 6 = 10 dozen.
The fall of Rs 10 raised market demand by 5 dozen.

Answer: Market demand is 5 dozen at Rs 20 and 10 dozen at Rs 10; adding buyers makes the market more responsive than any one buyer.

Memory hook: Add across buyers.

Tricks to remember Demand and Supply: Laws, Schedules and Determinants

Short memory aids. Say each one aloud once, then use it on a question the same day.

D-down, S-up
Demand slopes Down; Supply slopes Up.
Use it when: Shapes of curves.
Substitute vs complement
Substitutes: swap (tea or coffee); Complements: combine (ticket and popcorn).
Use it when: Related goods.
Along or shift?
Own price: slide along. Anything else: shift the line.
Use it when: Event questions.

Demand and Supply: Laws, Schedules and Determinants: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Which pair? · 1 mark · MCQBoard level

Cars and petrol are an example of:
(a) substitutes
(b) complements
(c) public goods
(d) monopolies

Try it first, then show the solutionSolution

Goods used together are complements (1 mark).

Answer: (b) complements

Example 2: Third mango (2 marks) · 2 marks · Very short answerBoard level

Srivalli buys 1 kg of mangoes at Rs 150 but 3 kg at Rs 50. Use diminishing marginal utility to explain why she needs a lower price to buy more. (2 marks)

Try it first, then show the solutionSolution

Each extra mango gives her less additional satisfaction (1 mark).
So she is willing to pay less for additional units, giving a downward-sloping demand curve (1 mark).

Answer: Each extra kilogram of mangoes gives Srivalli less additional satisfaction than the one before, so she is willing to pay less for more units. She therefore buys more only when the price falls, which gives a downward-sloping demand curve.

Example 3: Adding schedules (3 marks) · 3 marks · Short answerHarder board level

Individual demand for oranges at Rs 60 per kg is: P 1 kg, Q 2 kg, R 3 kg; at Rs 40 per kg: P 2 kg, Q 4 kg, R 5 kg. Find market demand at each price and comment on the effect of the price fall. (3 marks)

Try it first, then show the solutionSolution

At Rs 60: 1 + 2 + 3 = 6 kg (1 mark).
At Rs 40: 2 + 4 + 5 = 11 kg (1 mark).
Price fall of Rs 20 raises market demand by 5 kg: the law of demand (1 mark).

Answer: At Rs 60 market demand is 1 + 2 + 3 = 6 kg; at Rs 40 it is 2 + 4 + 5 = 11 kg. A fall of Rs 20 raises market demand by 5 kg, which agrees with the law of demand: a lower price increases the quantity demanded.

Example 4: Assertion-reason: complements · 1 mark · Assertion-reasonHarder board level

Assertion (A): If the price of cinema tickets rises sharply, the demand for cinema popcorn may fall.
Reason (R): Cinema tickets and popcorn are complements.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.

Try it first, then show the solutionSolution

Complements are used together; when one gets dearer, demand for the other falls, so R explains A (1 mark).

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Example 5: Shift or move? (5 marks) · 5 marks · Long answerHOTS / competency

Classify each event as a change in demand or supply, and say whether the curve shifts right or left: (i) coffee becomes much dearer and a family buys more tea; (ii) a cold-storage unit is built near mango farms; (iii) wholesalers expect potato prices to rise next month and hold back stock now; (iv) a rise in the price of mangoes reduces the quantity bought. Explain each. (5 marks)

Try it first, then show the solutionSolution

(i) Demand for tea shifts right: coffee is a substitute (1 mark).
(ii) Supply of mangoes shifts right: technology lowers losses and costs (1 mark).
(iii) Supply shifts left now: future price expectations (1 mark).
(iv) Movement along the demand curve: own price changed (1 mark).
Name the determinant in each (1 mark).

Answer: (i) A rise in the price of a substitute shifts the demand for tea to the right; the determinant is the price of a related good. (ii) Cold storage is technology that raises supply, so the mango supply curve shifts right. (iii) Holding back stock because of expected higher prices shifts current supply to the left; the determinant is future expectations. (iv) A change in the mango's own price moves the buyer along the demand curve, a fall in quantity demanded and not a shift.

Example 6: Case: the sweet shop · 4 marks · Case-basedHOTS / competency

Read the case and answer the questions.

A sweet shop sells 200 boxes a day at Rs 300 per box in normal weeks. Before a major festival it sells 450 boxes a day at the same Rs 300 per box. The week after the festival, sales fall to 120 boxes at the same price. The owner also notices that when a rival shop cut its price, his sales dropped.

(i) Which determinant explains the rise before the festival? (1)
(ii) What is the relationship between the two shops' sweets? (1)
(iii) Explain the fall after the festival and the effect of the rival's price cut. (2)

Try it first, then show the solutionSolution

(i) Seasonality (1 mark).
(ii) Substitutes (1 mark).
(iii) After the festival, seasonal demand ends and demand shifts left; when the substitute became cheaper, buyers moved to it, reducing demand for his sweets (2 marks).

Answer: (i) Seasonality. (ii) They are substitutes. (iii) After the festival the season's extra demand ends, so demand shifts to the left. When the rival cut its price, buyers moved to the cheaper substitute, so demand for his boxes fell at every price.

Where students lose marks in Demand and Supply: Laws, Schedules and Determinants

The slips examiners see most often, and what to write instead.

Mistake: Mixing "demand" and "quantity demanded"
Similar words.
Wrong working
A fall in price increases demand.
Right working
A fall in price increases quantity demanded; demand shifts only if another determinant changes.
Do this: Use "quantity demanded" for price effects.
Usually costs: 1-2 marks.
Mistake: Swapping substitutes and complements
Thinking of "related" only.
Wrong working
Tea and sugar are substitutes.
Right working
Tea and coffee are substitutes; tea and sugar are used together.
Do this: Test: swap or combine?
Usually costs: 1 mark.
Mistake: Not adding all columns
Skipping a buyer.
Wrong working
Market demand at Rs 100 = 2 + 4 = 6 kg.
Right working
2 + 4 + 6 = 12 kg.
Do this: Count buyers before adding.
Usually costs: 1 mark.
Mistake: Dropping "other things constant"
Omitting the condition.
Wrong working
Price up always lowers demand.
Right working
Price up lowers quantity demanded when other factors stay the same.
Do this: Add the condition.
Usually costs: 1 mark.

Exam tips

Write "other things constant" in every law-of-demand or supply answer.
In schedule questions, show the addition for each price row to earn method marks.
Use the words "shift" and "movement along" correctly; examiners check them.
Learn NCERT's examples: tea and coffee, tickets and popcorn, drip irrigation and cold storage.

Practice questions on Demand and Supply: Laws, Schedules and Determinants

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
When the price of a good rises and its quantity demanded falls, it shows:
(a) law of supply
(b) law of demand
(c) equilibrium
(d) monopoly
Hint
Inverse.
Answer
(b) law of demand
Q2 · 2 marks · Very short answerBoard level
Define supply and state the law of supply. (2 marks)
Hint
Willing and able.
Answer
Supply is the quantity sellers are willing and able to offer at a particular price. The law of supply says that as price rises, quantity supplied rises, and as price falls, it falls.
Q3 · 3 marks · Short answerHarder board level
At Rs 100 per kg, sellers A, B and C offer 2 kg, 4 kg and 6 kg; at Rs 150 per kg they offer 3 kg, 7 kg and 8 kg. Find market supply at each price and the change between them. (3 marks)
Hint
Add across sellers.
Answer
At Rs 100: 2 + 4 + 6 = 12 kg. At Rs 150: 3 + 7 + 8 = 18 kg. A Rs 50 rise in price raises market supply by 6 kg, in line with the law of supply.
Q4 · 1 mark · Assertion-reasonHarder board level
Assertion (A): If people expect the price of a good to fall next month, they may postpone buying.
Reason (R): Future price expectations are a determinant of demand.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Hint
Diwali discounts.
Answer
(a) Both A and R are true, and R is the correct explanation of A.
Q5 · 5 marks · Long answerHarder board level
Describe any four determinants of demand other than the good's own price, with an example of each. (5 marks)
Hint
Related goods, income, tastes, population, seasonality, expectations.
Answer
Prices of related goods: if coffee gets dearer, demand for its substitute tea rises. Income: higher income raises demand for better-quality goods. Population size and composition: more children raise demand for sports shoes. Seasonality: sweaters sell more in winter. Future expectations: buyers postpone purchases if prices are expected to fall. Tastes and preferences also matter. Each of these shifts the demand curve even if the own price stays unchanged.
Q6 · 3 marks · Short answerHOTS / competency
Suppose a new drip-irrigation technology lowers the cost of growing vegetables. Predict its effect on the supply curve and on the quantity supplied at the old price, giving reasons. A classmate says this is a movement along the curve. Is she right? (3 marks)
Hint
Technology is a determinant.
Answer
Lower costs raise profit at every price, so producers offer more at the old price: the supply curve shifts to the right. Your classmate is wrong: a movement along the curve occurs only when the good's own price changes. Here a determinant other than price (technology) has changed, so the whole curve shifts.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.What does the law of demand state?Show answer
Price up, quantity demanded down, other things constant.
2.What is market demand?Show answer
The sum of all individual demands at each price.
3.Give a pair of substitutes.Show answer
Tea and coffee.
4.Give a pair of complements.Show answer
Cinema tickets and popcorn (or cars and petrol).
5.What does technology do to supply?Show answer
It lowers costs and raises supply.

Demand and Supply: Laws, Schedules and Determinants: frequently asked questions

What is demand?

The quantity of a product people are willing and able to buy at a particular price.

What is the law of supply?

As price rises, quantity supplied rises, and as price falls, it falls.

What is the difference between substitutes and complements?

Substitutes replace each other; complements are used together.

What shifts the demand curve?

Changes in related goods' prices, income, tastes, population, seasonality and future price expectations.